Ultracold Quantum Gases Group

Welcome to the ultracold quantum gas research group at Aarhus University!

In our research we investigate the properties of atomic gases at extremely low temperatures. This allows us to understand the fundamental quantum mechanical behaviour of few- and many-particle systems.


Polaron energy across resonant interactions

Analyzing the Bose Polaron Across Resonant Interactions

Recently, two independent experiments reported the observation of long-lived polarons in a Bose-Einstein condensate, providing an excellent setting to study the generic scenario of a mobile impurity interacting with a quantum reservoir. Here, we expand the experimental analysis by disentangling the effects of trap inhomogeneities and the many-body continuum in one of these experiments. This makes it possible to extract the energy of the polaron at a well-defined density as a function of the interaction strength. Comparisons with quantum Monte-Carlo as well as diagrammatic calculations show good agreement, and provide a more detailed picture of the polaron properties at stronger interactions than previously possible. Moreover, we develop a semi-classical theory for the motional dynamics and three-body loss of the polarons, which partly explains a previously unresolved discrepancy between theory and experimental observations for repulsive interactions. Finally, we utilize quantum Monte-Carlo calculations to demonstrate that the findings reported in the two experiments are consistent with each other.

Read our paper on arXiv.


Atom number fluctuations in a BEC

Observation of Atom Number Fluctuations in a Bose-Einstein Condensate

Fluctuations are a key property of both classical and quantum systems. While the fluctuations are well understood for many quantum systems at zero temperature, the case of an interacting quantum system at finite temperature still poses numerous challenges. Despite intense theoretical investigations of atom number fluctuations in Bose-Einstein condensates (BECs), their amplitude in experimentally relevant interacting systems is still poorly understood. Moreover, technical limitations have prevented their experimental observation to date. Here we report the first observation of these fluctuations. Our experiments are based on a stabilization technique, which allows for the preparation of ultracold thermal clouds at the shot noise level, thereby eliminating numerous technical noise sources. Furthermore, we make use of the correlations established by the evaporative cooling process to precisely determine the fluctuations and the sample temperature. This allows us to observe a telltale signature: the sudden increase in fluctuations of the condensate atom number close to the critical temperature.

Read our paper on arXiv.


Loss spectroscopy reveals the temperature dependence of an Efimov resonance.

Temperature dependence of an Efimov resonance in 39K

Ultracold atomic gases are an important testing ground for understanding few-body physics. In particular, these systems enable a detailed study of the Efimov effect. We use ultracold 39K to investigate the temperature dependence of an Efimov resonance. The shape and position of the observed resonance are analyzed by employing an empirical fit, and universal finite-temperature zero-range theory. Both procedures suggest that the resonance position shifts towards lower absolute scattering lengths when approaching the zero-temperature limit. We extrapolate this shift to obtain an estimate of the three-body parameter at zero temperature. A surprising finding of our study is that the resonance becomes less prominent at lower temperatures, which currently lacks a theoretical description and implies physical effects beyond available models. Finally, we present measurements performed near the Feshbach resonance center and discuss the prospects for observing the second Efimov resonance in 39K.

Read our published paper at PRA or on arXiv.


Schematic of the optical setup and the local probing scheme.

Spatially-selective magnetometry of ultracold atomic clouds

We demonstrate novel implementations of high-precision optical magnetometers which allow for spatially-selective and spatially-resolved in situ measurements using cold atomic clouds. These are realised by using shaped dispersive probe beams combined with spatially-resolved balanced homodyne detection. Two magnetometer sequences are discussed: a vectorial magnetometer, which yields sensitivities two orders of magnitude better compared to a previous realisation and a Larmor magnetometer capable of measuring absolute magnetic fields. We characterise the dependence of single-shot precision on the size of the analysed region for the vectorial magnetometer and provide a lower bound for the measurement precision of magnetic field gradients for the Larmor magnetometer. Finally, we give an outlook on how dynamic trapping potentials combined with selective probing can be used to realise enhanced quantum simulations in quantum gas microscopes.

Read our paper on arXiv.


Monopole oscillation frequency of a Lee-Huang-Yang fluid.

Dilute Fluid Governed by Quantum Fluctuations, published in PRL

Understanding the effects of interactions in complex quantum systems beyond the mean-field paradigm constitutes a fundamental problem in physics. Here, we show how the atom numbers and interactions in a Bose-Bose mixture can be tuned to cancel mean-field interactions completely. The resulting system is entirely governed by quantum fluctuations - specifically the Lee-Huang-Yang correlations. We derive an effective one-component Gross-Pitaevskii equation for this system, which is shown to be very accurate by comparison with a full two-component description. This allows us to show how the Lee-Huang-Yang correlation energy can be accurately measured using two powerful probes of atomic gases: collective excitations and radio-frequency spectroscopy. Importantly, the behavior of the system is robust against deviations from the atom number and interaction criteria for canceling the mean-field interactions. This shows that it is feasible to realize a setting where quantum fluctuations are not masked by mean-field forces, allowing investigations of the Lee-Huang-Yang correction at unprecedented precision.

See also this news announcement.

Read our published paper in Physical Review Letters or on arXiv.


Fabrice Gerbier

Fabrice Gerbier visits

Researcher Fabrice Gerbier from CNRS in France is visiting the department. He has recently studied the spacial coherence in a superfluid gas of bosonic atoms in an optical lattice. For independent atoms excited by a near-resonant laser, absorption-emission cycles destroy spatial coherences related to diffusion in momentum space. For strongly interacting bosons, Fabrice observed an anomalously slow coherence due to clustering of atoms.

Link to official lecture event.


Gabriele Ferrari

Gabriele Ferrari visits

Researcher Gabriele Ferrari from Trento is visiting the department. By rapidly crossing the critical temperature to Bose-Einstein condensation, he has studied the growth of boundary defects, related to the Kibble-Zurek mechanism. These defects are identified as quantized vortices, and in his research, Gabriele has studied their real-time dynamics and interactions.

Link to official lecture event.


Mick´s PhD defense - Atom Number Jumps in Ultracold Clouds

During his studies, Mick Althoff Kristensen has studied atom clouds cooled to ultra low temperatures. When a cloud of atoms is cooled to the lowest temperatures found anywhere in the universe, their quantum mechanical nature reveals itself. A prime example is the Bose-Einstein condensate, where the atoms accumulate in the quantum mechanical ground state and form a single large quantum object. Mick Althoff Kristensen has developed the most stable source of Bose-Einstein condensates, which has allowed him to study how atoms jump in and out of the condensate.

Official announcement.


Experimental setup and procedure for probing the BEC phase transition

Measurement-enhanced determination of BEC phase transitions

We demonstrate how dispersive atom number measurements during evaporative cooling can be used for enhanced determination of the parameter dependence of the transition to a Bose–Einstein condensate (BEC). In this way shot-to-shot fluctuations in initial conditions are detected and the information extracted per experimental realization is increased. We furthermore calibrate in situ images from dispersive probing of a BEC with corresponding absorption images in time-of-flight. This allows for the determination of the transition point in a single experimental realization by applying multiple dispersive measurements. Finally, we explore the continuous probing of several consecutive phase transition crossings using the periodic addition of a focused 'dimple' potential.

Read our published paper or the arXiv version.


Workshop banner

Center for Quantum Optics and Quantum Matter hosts conference on quantum simulations

The concept of quantum simulation is currently being realised in an increasing range of physical systems with expanding scope and success. Neutral atoms and photons are among the most promising building blocks of quantum simulators, owing to the remarkable degree to which they can nowadays be controlled in the laboratory. This workshop will explore the frontier of these exciting developments. It will thereby provide opportunities to identify current challenges and new directions in the field of quantum simulations by sharing latest results and ideas across different platforms, ranging from photonic settings and polariton systems to cold atom experiments. By bringing together experts on this range of topics, the workshop aims to provide an inspiring venue for discussing common interests and future perspectives for experiments as well as theory of quantum-many body systems in and out of equilibrium.

The workshop is organized jointly by the Center for Quantum Optics and Quantum Matter (CQOM) at Aarhus University and the Institute for Theoretical Atomic, Molecular and Optical Physics (ITAMP) at the Harvard-Smithsonian Center for Astrophysics.

See more on the workshop homepage.


Professor Thomas Killian

Thomas Killian visits

Professor Thomas Killian from Rice University is visiting the department. He has recently observed polaron physics by immersing a Rydberg atom in a Bose-Einstein condensate. Moreover he is developing experimental techniques for studying strongly correlated plasmas using ultracold atoms.

Link to official lecture event.


Jan Arlt

New grant: Quantum simulation of quasiparticles

Georg Bruun and Jan Arlt have received a new grant from the Danish Council for Independent Research: Quantum simulation of quasiparticles.

An announcement is available in danish here.


Scattering properties of K and Rb.

Time-of-flight expansion of binary Bose-Einstein condensates at finite temperature - published in New Journal of Physics

Ultracold quantum gases provide a unique setting for studying and understanding the properties of interacting quantum systems. Here, we investigate a multi-component system of 87Rb–39K Bose–Einstein condensates (BECs) with tunable interactions both theoretically and experimentally. Such multi-component systems can be characterized by their miscibility, where miscible components lead to a mixed ground state and immiscible components form a phase-separated state. Here we perform the first full simulation of the dynamical expansion of this system including both BECs and thermal clouds, which allows for a detailed comparison with experimental results. In particular we show that striking features emerge in time-of-flight (TOF) for BECs with strong interspecies repulsion, even for systems which were separated in situ by a large gravitational sag. An analysis of the centre of mass positions of the BECs after expansion yields qualitative agreement with the homogeneous criterion for phase-separation, but reveals no clear transition point between the mixed and the separated phases. Instead one can identify a transition region, for which the presence of a gravitational sag is found to be advantageous. Moreover, we analyse the situation where only one component is condensed and show that the density distribution of the thermal component also shows some distinct features. Our work sheds new light on the analysis of multi-component systems after TOF and will guide future experiments on the detection of miscibility in these systems.

Read our manuscript in New Journal of Physics or on the arXiv.


Zoran Hadzibabic

Zoran Hadzibabic visits

Professor Zoran Hadzibabic from Cambridge University is visiting the department. By constructing a box potential for ultracold atoms, he has recently made major contributions to the field. More specifically, the box potential has opened up for new studies both weakly an strongly interacting Bose gases, in and out of equilibrium.

Link to official lecture event.


Nils in the lab

Congratulations to Nils!

On the 23rd of February, Nils Byg Jørgensen successfully defended his PhD thesis "Observation of Bose Polarons in a Quantum Gas Mixture". The assessment committee consisted of Prof. Matthias Weidemüller from Heidelberg University in Germany and Researcher Matteo Zaccanti from the University of Florence. The thesis is currently available here. He will carry on his scientific work by continuing in the group as a postdoctoral researcher. (02/2018)

Matthias Weidemüller

Matthias Weidemüller visits

Professor Matthias Weidemüller from Heidelberg University is visiting the department. In his recent research, he has explored mass-imbalanced Li-Cs mixtures. These are especially well-suited for studies of heteronuclear Efimov physics, since the mass imbalance yields a scaling factor which allows observation of multiple consecutive Efimov resonances.

Link to official lecture event.


Matteo Zaccanti

Matteo Zaccanti visits

Researcher Matteo Zaccanti from LENS in Florence is visiting the department. In his research career, he has conducted important studies on KRb mixtures, Efimov states, Fermi polaron physics, and ferromagnetic Fermi gases and has thus made many important contributions to the field of ultracold gases.

Link to official lecture event.


Energy shift and decay rate of the polaron.

Finite-temperature behavior of the Bose polaron

After our recent observation of the Bose polaron, we are aiming to understand the quasiparticle in more depth.

Here we consider a mobile impurity immersed in a Bose gas at finite temperature. Using perturbation theory valid for weak coupling between the impurity and the bosons, we derive analytical results for the energy and damping of the impurity for low and high temperatures, as well as for temperatures close to the critical temperature Tc for Bose-Einstein condensation. These results show that the properties of the impurity vary strongly with temperature. The energy exhibits an intriguing non-monotonic behavior close to Tc, and the damping rises sharply close to Tc. We finally discuss how these effects can be detected experimentally.

Read our manuscript in Physical Review A or on the arXiv.



The Villum Foundation